Reactive, solvent-free adhesives cross-linkable with moisture from the air, particularly hot-melt adhesives, are well known in the state of the art. Such adhesives are prepared from low-molecular starting materials comprising OH groups, such as polyesters or polyethers, which are then converted to reactive isocyanate-terminated ahesives with a stoichimetric excess of isocyanates. 4,4′-diisocy-anato- diphenylmethane (MDI), 1,5-diisocyanato-naphthalene (NDI), 1,6-diisocyanato-hexane (HDI), 2,4-diisocyanato- toluene (TDI), 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (IPDI) or their prepolymers can be used as isocyanate components.
Alternatively, said reactive coating and adhesive materials can also be produced on the basis of a silane-functionalized polymer. The addition of catalysts, resins and other additives and adjuvants is usual.
Depending on the above starting materials and their average molecular weights and on the desired properties of the adhesive to be produced, the underlying polyaddition reaction can take several hours. Two or more adhesive components are brought together and are processed, under the effect of heat, to give a hot, liquid mass which, in this state, can be used as a hot-melt adhesive. The cross- linking reaction takes place by taking moisture from the air and/or from a wetted surface. This means that after their preparation, these hot-melt adhesive must be kept dry, with any moisture from the air being eliminated.
Therefore, for transportation and storage such an adhesive is filled into tightly sealable containers (e.g. 20-liter or 200-liter tin-barrels) and is stored in a dry state there, with any moisture from the air being eliminated. Before use, the cooled and generally solid adhesive must be melted out of these containers and conveyed or fed to the processing and application units. Melting is achieved, for example, by means of a barrel-melter (melting device) whose heatable piston provided with sealing rings or joints is brought under pressure into a container filled with said adhesive. By means of a gear-type pump installed in the piston, the molten and viscous to fluid hot-melt adhesive is conveyed or fed through heated conduits or pipes to an application unit. Depending on the size of the container and on the adhesive formulation, the melting capacity of such a barrel- melter is in the range of about from 20 to 80 kg/hour.
In some fields of application such a melting capacity is, however, not sufficient, particularly not for the coating of flat surfaces. In addition, when relatively small barrels are used, it is impossible to achieve substantially continuous operation because of frequent changing of said barrels. Besides, the purchase of a barrel melter by the user means a considerable capital investment. If the container which contains the hot-melt adhesive becomes leaky during transportation, the adhesive can rapidly become unusable, particularly if it is exposed to moisture. Another disadvantage is that after opening the container, the adhesive must be processed completely and in a relatively short time because any adhesive resi- dues become unusable relatively rapidly.
Another disadvantage of the state of the art described before is the restriction that the use of such systems is limited exclusively to reactive adhesives only. The alternating processing of reactive adhesives and thermoplastic adhesives (based on EVA, APAO or PA) is disadvantageous for both economical and technical reasons.
The use of multicomponent systems in the production of hot-melt adhesives and coating materials is generally known per se. A feature common to the systems disclosed in EP 0 304 026 B 1, in the Handbook of Adhesives, 2nd Edition, p. 581 ff and in the ICI polyurethanes book, 2nd Edition, p. 93 ff, is that the com- ponents exclusively react with one another and that, after the reaction has taken place, they no longer have any reactivity for secondary reactions, e.g. with moisture. Also, the components have necessarily to be used and consumed directly after mixing.